The present invention relates to a keel structure in connection with the hull of a nautical vessel, such as a sailboat.
In principle, there are two basic keel designs: a fin keel which is fastened to the hull of a boat and can also be liftable, and a displacement keel in which the hull of a boat is expanded to form a keel fin. It is obvious that the hull may include several keels. The keel structure usually contains an additional ballast, such as iron or lead.
A purpose of the keel especially on a sailboat is to counterbalance the effect of a lifting lateral force directed to the hull of a boat from the sails as well as to counteract the inertia of a boat. Similar functions are also assigned to a possible additional fin and to the rudder. The main purpose of the rudder is, however, to provide the lateral force required for steering the boat. The deeper or heavier the keel, the greater the boat-trueing moment. In addition to the above, the practical dimensioning of a keel is effected by the strength required of the keel, particularly at the junction of the hull and the keel against momentary dynamic lateral forces, With the exception of light jollies, in which the main purpose of the keel is to prevent the leeway, the crew themselves acting as a counterforce to the force exerted by the sails, the keel must be designed to be more or less wide in the crosswise direction of the hull. Thus, the keel should be designed in such a manner so as to be minimized that the drag caused thereby would be as little as possible. The presently used designs aim to minimize the turbulence of by-pass current and the friction surface (wet surface). A preferred shape of the keel keel must naturally be as streamlined as possible.
Since water is an incompressible fluid, the keel of a sailboat acts the same way as a displacement pump, such that the effect taken up thereby (without friction effect) is a function of the largest cross-sectional area of a keel, the mean draft thereof and the speed of a boat.
The displacement pumping effect taken up by the boat has an effect on the speed of the boat particularly at low speeds, whereby other features of the boat, such as the length of the hull, have no effect on its speed.
Hereinbelow is a list of the components which build up the advancing resistance of a boat:
the pumping effect developed by the displacement of the keel and the hull of a vessel, which effect depends on the largest cross-sectional area of the advancing direction of a boat, the mean draft of this cross-sectional area, and the speed of the boat,
the advancing resistances of displaced water masses (the rise of water masses to water surface),
the friction between the hull and the keel of the vessel and water, and
the air drag caused by the above-water components and structures of the vessel.
It can be shown by calculations that the most important of these are the displacement pumping effect and the flow losses of displaced water masses.
At present, design of the hull and keel of a vessel has to a great degree developed experimentally. Experimentally well-found designs have been used and theoretical basis is generally poor. Flow analyses are highly complex even though the question is about an incompressible fluid. It should also be noted in practice that the disturbance components caused by the waves complicate calculations. Thus, it can be concluded that presently, the designing and constructing of sailboats and in particular their keels are to a great degree based on traditional and previously used structural possibilities and minor improvements thereof. No attention has been paid to the actual basic problem, that is the displacement pumping effect caused by the displacement of a boat.
The crosswise surface area of the displacement of the hull and keel of a vessel relative to travelling direction can be divided into two components: hull and keel (also fin/fins and rudder). With a displacement type of keel, the keel is determined so as to begin at that point on a hull at which the inclination of bottom exceeds 45.degree.. The amount of water displaced by the hull of a vessel is usually greater than that displaced by the keel. The displacement pumping effect caused by the hull of a vessel is not dealt with in this invention. The largest surface area of a keel (fin/fins and rudder) transverse to the travelling direction of a vessel is generally substantially smaller than the corresponding transverse surface area of the hull of a vessel. It is obvious, however, that the mean drafts of these surface areas are different. The mean draft of the transverse hull area of a vessel is considerably lower than the mean draft of the transverse keel area. Thus, the relative significance of the keel in terms of displacement pumping effect increases, since the required lifting distance is greater.
Thus, it is important that the displacement pumping effect in a keel structure be taken into consideration and, for this part, the displacement pumping effect can be eliminated totally or partially by means of controlled steering of water. In this context, it is possible to refer to pump analogy, the total lifting height of a pump being determined by the fall between the end of a pressure-side pipe and the pump regardless of the shape of the pipe. This, of course, on the condition that a flow friction of the pipe is ignored.
Publications WO-82/00447 and WO-83/00129 can be cited for describing the prior art. The former discloses a solution for decreasing the advancing resistance of a vessel by narrowing the sides of the hull of a vessel. The solution is based on the fact that, as it increases, the speed of a vessel with reach a range wherein the hull length of a vessel determines a so-called critical speed for the boat. By narrowing the sides of a hull it is possible to produce a more preferable surface wave and hence a more favorable speed within the critical speed range of a boat. On the other hand, the power demand of a displacement pumping effect caused by the keel is linear with the speed of a vessel. Most benefits in the solution are obtained at low speeds since other resistances or drags of the vessel are exponential relative to speed. The latter primarily deals with a rudder structure which is hollow in a manner that the cross-sectional area diminishes from forward edge to trailing edge. The side of a rudder, adjacent to the trailing edge, is provided with an array of holes whose purpose is to provide an improved flow to the draft side of a rudder in order to avoid flowing disturbances on the draft side of a rudder. A premise in the latter solution is also that incompressible water behaves the same way as compressible air.